Reinforced concrete



p 1941- F. EMPERGER REINFORCED CONCRETE Filed Fb. a, 1940 InrazZo j" fore had to be made of high quality steel.

. steel.

Patented Sept. 2, 1941 namroncnn coNcnE'ra Fritz Emp'erger, Vienna, Germany, assignor of ten per cent'to Joseph 0. N. Y.

, Application Febrilary s, 1940, "Serial Germany January 25, 193

7 Claims.

The invention refers to objects or structures of reinforced concrete, such as girders, arches, frames, beams, and parts thereof.

The concrete of structures of such type is subject to cracks in zones where tensile stresses occur under load, and it has beensuggested to prevent formation of such cracks and to increase the strength of the structure by preliminary or prestressing all the reinforcements which there- Considerable difliculties were met however, in manufacturing such concrete bodies or structures either in a plant or in situ, i. e. at the place where they were to be used, in that uniformly prestressing a great number ofreinforcements requires complicated apparatus and great care in their use, and the great number of relatively thin reinforcements of high quality steel considerably increased the cost.

Taking a cylindrical reinforcement embedded in concrete, it has been shown (of. O. Graf Beton und Eisen, 1910, p. 177 and "Handbuch fiir Eisenbetonbau," 4th ed., first vol., p. 40) that the increase in plasticity of the concrete by reinforcement is very small, and the less the thicker the layer or body of concrete covering the reinforcement is. The maximum plastic or elastic deformation of a layer of 2 mm. thickness around the cylindrical reinforcement has beenfound to be 0.4% at most before rupture occurs,.

and if the thickness of the covering cylindrical layer or body amounted to30 mm., the maximum elongation without rupture has been found to be only 0.2%. Taking a steel the elongation of which amounts to 0.2% at a stress of 400 kg./sq. cm., the covering layer of 30 mm. thickness will yield and form cracks if that stressof 400 kg./sq. cm. is exceeded.

Taking however a higher quality steel the elongation of which amounts to 1.4% at a stress of 2800 kg./sq. cm., experiments made by the inventor have shown that a relatively thin cover of concretewill be suiiiciently plastic so as to crack only at It has been suggested therefore to use high quality steel for reinforcements of concrete and 'to arrange them in such numbers and proximstressed reinforcements through the bond upon the yield point of that high quality Ollier, New York,

the concrete were so ,high that they counterbalanced the tensile stresses exerted upon the concrete by the predetermined maximum load.

It has been foundhowever that crack formation is not dangerous as long as cracks, when formed, are preventedfrom widening in continuous use and under recurrent load, and substantially close when the load is moved.

Hence it follows that the reinforcements-should be prestressed only to such an extent that the compressive stresses exerted by them upon the concrete bonded with them suffices to substantially close the cracks under dead load.

Even if prestressing is limited in this way, or highly prestressed reinforcements are arranged in such distance from each other that the compressive stresses produced by them in the con-. crete bonded with them sufflce, to substantially close cracks produced by the live load, their numher is still great and the difficulties as to uniformly prestressing them as well as their high cost persist.

It is therefore an object of theinvention to reduce the cost of manufacturing in a plant or in forcements.

- situ of objects or structures of reinforced concrete the hair line-cracking of which is reduced by prestressing those reinforcements.-

It isanother object of the invention to increase the strength andreduce the. crack formation of objects or structures of reinforced concrete in which the reinforcements are pre stressed. I

It is still another object of the invention to secure moreuniform prestressing of those reinformed under predetermined maximum live load and causing the cracks to substantially close under dead load by the use of prestressed reinforcements.

It is still a further object of the ihvention to prevent 'hair line cracking in reinforced concrete structures.

It is still another object of the invention to improve the property, in particular the tensile strength of the concrete used in reinforced concrete structures.

According to the invention main reinforcements or rods are used and disposed in the usual be arranged manner, and in addition thereto prestressed othnumber of additional reinforceeifected than preuniform prestressing of the wherever it is suitable. After the main reinforcements or rods have been placed in the mold or form and the additional preferably rod-like reinforcements uniformly prestressed, the concrete is poured or pressed into the mold, tightly around all those reinforcements, and allowed to set and shrink, whereby a tight and firm bond between the concrete and all the reinforcements is efiected.

stressing of all the reinforcements of high quality steel. It is also possible to out these additional reinforcements so as to cover only regions or zones in which tensile stresses under maximum load occur which might cause dangerously wide, or gaping cracks. In particular, the cut pieces of the additional reinforcements can be arrang in any desired manner and relative configuration so as to follow e. g. a curved or polygonal structure and its axis.

- The objects and nature of the invention will be more clearly understood when the specification' proceeds with reference to the drawing in which Fig. 1 shows more schematically in a section taken in the direction of the reinforcin bar-s or rods the arrangement of main reinforcements and prestressed additional reinforcements in a corner of a reinforced concrete frame or arch; Figs. 2 and3 show each a cross section through the web of a T-girder, the prestressed additional reinforcements being arranged in the lower portion or the corners of the web in which hair line cracks are most likely to form: Fig. 4 shows more schematically a method and means for preliminary or prestressing the additional bars or rods positioned in a form for molding a reinforced concrete beam; and Fig. 5 shows a cross section taken along a main reinforcement through a piece of concrete in which twisted or plaited, prestressed additional bars or wires are used to form a lattice work or ttin rtefe rring to Fig. 1, the corner portion of the mold or form in which the reinforced concrete structure forming a frame or arch is to be made is shown in cross section. Bent and otherwise suitably shaped main reinforcements or bars preferably of ordinary steel are shown therein in elevation. The position of additional preferably rod-like reinforcements of high quality steel which are to be prestressed is shown in dotted Thus, additional reinforcements should in the horizontal planes dotted lines or-a near the bottom and b-b near the top of the girder or beam forming the upper part of the frame or arch. Other additional reinforcements should be arranged in the plane d-d in the upright forming the lateral part of h. Additional reinforcements ma the corner, the additional reinforcements thus shown resulting in a polygonal arrangement composed of groups of additional reinforcements arranged in different planes. Removable abutv ments on the mold can be provided for prev stressing the additional-reinforcements, as indicated in Fig. 1; may also be anchored in the foundation, or

After the prestressed additional reinforcements have thus been firmly bonded with the concrete so that the prestress applied to them is maintained Y in them by that bond, the removable. means, if any, for prestressing them relative to the form or mold, as

The main reinforcements preferably consist of rods of ordinary steel as heretofore used forthis purpose and have in any case a lower yield point and tensile strength than the additional reinforcements.

The total area of the cross sections of the main reinforcements is Fe (also designated with Al): the total area of cross sections of the additional reinforcements is Fe (also designated The total area of all the reinforcements, main and-additional, Fe=FJ+Fe".

The main reinforcements have a yield point of 1' which is considerably lower than the yield point I, of the additional reinforcements; Calling the average yield point of the two kinds of steel used for the main and additional reinforcements 'y prestressing of the additional reinforcements should be effected to such an extent at the maximum that desired safety, and preferably double safety is obtained with respect to that average yield point 1,

Although the additional reinforcements add to the overall strength of the reinforced structure and their presence and cross sections, etc., are to be duly taken into consideration in calculating the structure, it should be understood that the prestressing of the additional reinforcements in the main serves to reduce the width of the hair line cracks if and when they occur undenpredetermlned maximum or permissible load, or to prevent their coming into existence.

As stated above, prestressing, of reinforce ments embedded in and thereby bonded with the set and shrunk concrete results in compressive stresses within thebody of the concrete surrounding each individual prestressed reinforcement, the absolute amounts of those compressive stresses forming amaximum close to the reinforcement and quite rapidly decreasing with the distance from that prestressed reinforcement.

Referring to Fig. 2 it will be appreciated therefore that the compressive stresses imparted to the concrete body or layer surrounding each intov the prestressed reinforcements dividual additional reinforcement indicated by Fe" will reach an adjacent main reinforcement indicated by cross hatching and reference character Fe, it being understood that the number of those additional reinforcements and their dis-. tance from the main reinforcements be such that this result can be obtained. Therefore, if in the concrete of that cross section tensile stresses are produced by a live load, they will be counteracted by those compressivestresses, and if the tensile stresses thus imparted to that cross section are greater than the compressive stresses and therefore eventually result in hair line cracks, the width of the latter will be considerably smaller thanit would be if no prestressed additional reinforcements were present. Hence it follows well as the latter, can be removed.

with Ae i'.

that by the use of main reinforcements and a relatively small number of additional prestressed reinforcements of a steel of higher quality than the main reinforcements, furthermore by arranging the prestressed additional reinforcements in suitable proximity to the main reinforcements-so that the compressive stresses are transmitted up to the main reinforcements, and finally by arranging the prestressed additional reinforcements in regions of the cross section and in. suitable relation to a surface most endangered by tensile stresses caused therein by the load, the formation of hair line cracks of danserous or undesirable size and width can successfully be prevented. The number of addi- 1.5

tional reinforcements and the extent of their prestressing can easily be calculated or ascertained by a few experiments in each individual-case, and the maximum preliminary stress put on the adthus admitted moisture into the'interio r 'of the,

ditional reinforcements should be such that an average double safety orany other desired or prescribed safety of the structure is secured.

It will furthermore be appreciated from the showings of, Figs. 2, 3" and 5 that the cross section per sq. cm. inthat area does not exceed 50 to 75kg, the tensile strength of the concrete in that area can be taken into consideration in addition to that of the total tensile strength of the and by and (by the cracks extended from the outer surface of the stressed portion of the structure to its neutral axis. They gaped widely and structure which corroded the reinforcements and weakened the concrete. By arranging additional reinforcements according to the invention, parof the individual rod-like main reinforcements is advantageously considerably larger than that of the additional rod-like reinforcements and consequently the total area Fa will very considerably exceed the total area Fe The load, dead as well as live, will therefore be carried in the main by the main reinforce ments. As to the prevention of-dangerous crack formation, or reduction of the size of the cracks formed under live load, to a desired extent, the

prestressing of the additional reinforcements comes decisively into play.

Besides the number of individual additional reinforcements, also their tensile'strength and yield point has to be taken into consideration.

Corners of reinforced concrete structures are particularly ubject to hair line cracking and should be re, forced inthe way to be derived from Fig. ,,3 without further comment.

Though anybody skilled in the art is capable of applying the invention in practice, it should be mentioned that according to tests carried out j by the inventor, high quality steels canbe successfully used for additional reinforcements havticularly in the endangered zones of the most stressed cross section and near its outer surface, such dangerously gaping cracks can either be prevented or their width under predetermined load be reduced. It is a preferred feature of the invention to measure the degree .of prestress on the additional reinforcements, furthermore to choose their number and'distance from the main reinforcements so that under dead load the compressive forces produced by the prestressed additional reinforcements suflice to substantially close the cracks when formed under predetermined maximum live load.

Since the compressive stresses produced in the l bodyof the concrete closely surrounding the prestressed additional reinforcements quasi improve the properties of that concrete in the way de-- ing a safe tensile strength of 1000 to 1200 kg./sq.

cm., this value to be taken as a basis for calculation of their permissible prestressing.

Most surprisingly, the tensile strength of the concrete, as small as it maybe, can also be taken into consideration in calculating the structure according to the'invention. With normal concrete of present time manufacture about 20 to 30 kg./sq. cm. tensile strength can be assumed for a concrete mass undergoing bending by the live or deadload. Due to the compressive stresses imparted to the concrete according to 'the invention, tensile strengths up to 50 and 75 kg./sq. cm. can safely be made a basis for calculation; in other words, if tensile stresses imparted to the most stressed zone of a structure by the dead and live load do not exceed 50 to 75 kg. per sq. cm., the concrete will neither loosen from the reinforcements, nor crack. Now, in calculating the structure, its cross section has to be taken into consideration in which the largest tensile stress occurs under predetermined maximum load. It is common to consider in such case only the total tensile strength of the reinforcements positioned in the stressed area of that cross section. If however the tensile stress scribed above, it is even possible to use for the purposes of the invention concrete of lower quality than heretofore used for structures of same dimensions, purposes and loads. 0n the other hand, with that improvement in quality of the concrete according to the invention, reinforced concrete can be used for purposes heretofore unknown or only to be served by more complicated and expensive structures.

The degree of pres'tressing of the additional reinforcements depends upon the result to be obtained and particularly the maximum loads to be carried. .In many structures and particularly those in which the live load is relatively small compared with the dead load, a low degree of prestressing suflices, and in such cases also the quality of steel used for the additional reinforcements need not considerably exceed that used for the main reinforcements. Since the individual additional reinforcements are of considerably smaller cross section than the individual main reinforcements, wires can be used for the former. Due to-the manufacture of wires, their tensile strength and yield point is usually higher than those properties of the bars from which the wires have [been drawn. Therefore, if relatively low degrees of prestress are to be applied, it sufllces sometimes to use for the main reinforcements thicker rods and for the additional reinforcements drawn wires of substantially the same steel quality.

As pointed-flout above, all the reinforcements are placed in the mold or form, and prestresses are applied before the concrete is poured into the mold or form. However, it is within the realm of the invention to place all the reinforcementsin the mold and to apply the prestressing after the 'concretehasbeenplacedinthemold.itbeing 'understoodthatsuchprestressinghastobeperformed before the concrete starts to set.

Any means and method for-prestressing the additional reinforcements can be applied.

inforeedin the region of tension by rod-like reinforcements of steel and other rod-like reinforceleast within one of these loops a wedge k is positioned so that by driving it in, any desired degree of prestress can be applied to the additional reinforcements. After the concrete has been poured into the mold and finally set, the wedge or wedges are removed, the loops cut oil, the mold is removed, and any'still projecting ends of the pulling wires cut off. Instead of wedges, any other means for accurately efl'ecting prestressing can be used, such as tensometersor expansion metersz' also winches and screws can be used stressing the loop in Fig. 4. and by counting the number of revolutions of the screw the accurate stress desired can be applied.

In order to improve the bond between the concrete and the prestressed additional rein forcements, any means known in the art can be used. In particular, theadditional reinforcements substantially arranged in the direction of the tensile stresses produced in the concrete by the load may be made of two or more twisted or ments of superior tensile strength than and supplementing said first mentioned reinforcements, said other reinforcements ,arranged such distances from said first mentioned reinforcement and a surface under tension of the structure and being prestressed so that the compressive struses produced in the body of the concrete bonded with an individual other reinforcement extend to an adjacent first mentioned reinforcement and said surface thereby substantially reducing or preventing hair line cracking of' the concrete.

3. A beam, girder or frame structure, or part thereof, substantially consisting of concrete reinforced in the. region of tension by rod-like main reinforcements of steel and other rod-like reinforcements of superior tensile strength than and supplementing said main reinforcements, said other reinforcements being sufficiently prestressed and arranged such distances from a surface under tension of the structure subject to hair line cracking under live load so as to keep, by the compressive stresses produced in the concrete by said prestressed other reinforcements, within a predetermined limit the width of ,suchcracks when formed under predetermined maximum live load and to sustantially close those cracks when the live load is removed.

plaited wires, as' shown in Fig. 5, where the upper transverse wire illustrates a main reinforcement and the two plaited wires below an additional prestressedreinforcement. By arranging transverse wires passing the loops. of theplaits as shown in Fig. 5, a lattice work or netting of additional reinforcement can be obtained. The transverse wires can be rigidly connected with the plaits, for instance by welding. Such transverse wires at the end of the plaits can then be formed into loops and used for prestressing the plaits in a method as described with reference to Fig. 4. I

It will be appreciated from the above that by the invention many outstanding advantages are obtained as to saving of expensive high quality steel, more uniformity and ease of prestressing the'relatively small number of additional reinforcements, improvement of the properties of the concrete used, preventing the'formation or reducing the size and particularly avoidingdangerous widths of cracks, and prolonging the life of the final structure.

It should be understood that the invention is not limited to any exemplification or illustration I forcement extend to an adjacent main reinforcement.- 2. A beam, girder or frame structure, or part '4. A beam, girder or frame structure, or part thereof, substantially consisting of concrete reinforced in the region of tension by rod-like main reinforcements of steel and other rod-like reinforcements of superior tensile strength than said main reinforcements, said other reinforcements being prestressed and arranged such distances from said main reinforcements as to substantially improve by the thus produced, compressive stresses in the concrete the tensile strength of the concrete bonded with them and supplement said main reinforcements. f

, 5. A beam, girder or frame structure, or part thereof, substantially consisting of concrete reinforced in the region of tension by rod-like main reinforcements of steel and other rod-like reinforcements of superior tensile strength than said main reinforcements, said other reinforcements being prestressed and ar :u'. uch distances from said main reinforcements and a surface under tension of the structure so as to improve by the thus produced compressive stresses in the concrete the tensile strength of the concrete bonded with them, supplement said main reinforcements and substantially reduce or preven hair line cracking of the concrete.

6. A beam, girder or frame structure, or part thereof, substantially consisting of concrete re- .inforced'in the region of tension by main rods of steel and other rod-like reinforcements of superior tensile strength than andsupplementing said main rods, said other reinforcements exemplified by steel wires being of considerably smaller cross section than said main rods, said other reinforcements being prestressed and arranged such distances from said main rods and a surface under tension of the structure so as to substantially reduce or prevent hair line cracking of the concrete by the compressive stresses produced in the concrete bonded with said prestressed other reinforcements.

'l. A beam, girder or frame structure. or partthereof, substantially consisting of concrete reinforced in the region of tension by main rods of steel and other rod-like reinforcements of thereof, substantially consisting of'concrete re- 7 superior tensile strength than and supplementing of substantiallysmaller cross section than said main rods and combined in networks as exemplified 'by plaited strands extending substantially in the direction of tensile stresses produced in the concrete under load and wires interlaced with said strands, said wires extending in a. substantially vertical direction to said strands, said,

other reinforcements being sumciently prestressed 2,255,022 5 ing said main rods, said other reinforcements be-.

and arranged such distances from said main rods and a surface under tension or the structure as to substantially reduce or prevent hair line cracking of the concrete by the compressive stresses thus produced in the concrete and substantially extending to said adjacent main re inforcements and surface.

FRITZ EMPERGER. 

